A fuel vapor treatment apparatus includes a canister which temporarily adsorbs fuel vapor evaporated in a fuel tank. The fuel vapor described from the canister is introduced and purged, through a purge passage, into an intake passage of the internal combustion engine by intake pressure. Amount of air including the fuel vapor, which is purged into the intake passage, is controlled by a purge control valve disposed in the purge passage.
The purged fuel vapor is combated in the engine with the fuel supplied from a fuel injector. Hence, it is necessary to accurately measure the amount of fuel vapor in order to keep an air-fuel ratio within a predetermined range.
JP-5-18326A and JP-5-33733A (U.S. Pat. No. 5,216,995) show a system in which concentration of the fuel vapor is detected by use of a flowmeter. Since the flowmeter is disposed in the purge passage, the concentration of the fuel vapor can be detected only when the air including the fuel vapor described from the canister flows in the purge passage. In order to detect the concentration anytime, a concentration detecting passage can be branched from the purge passage to provide a concentration-detecting unit therein. However, in a case that the concentration detecting passage is connected to an atmosphere port of the canister, the fuel vapor that has returned to the canister through the concentration-detecting unit is adsorbed by the absorbent at a vicinity of the atmosphere port. At this moment, when the engine is stopped to terminate the purging of the fuel vapor, the fuel vapor returned to the atmosphere port is diffused into the atmosphere passage. One end of the atmosphere passage is connected to the canister, and the other end is opened to the atmosphere. Thus, the fuel vapor which has returned to the atmosphere port could have been discharged into the atmosphere through the atmosphere passage.